Field of the Invention
The invention relates to a method for controlling the power of a fuel cell stack, a method for controlling the power of a drive unit of an electric vehicle, and a fuel cell device.
A system for controlling the power of a fuel cell stack in which the cell voltage of the last one of the fuel cells is adjusted according to given requirements is known. A drawback of this system is that the cell voltage of the majority of the fuel cells is not taken into account. Furthermore, it is known to control the power of the fuel cell by controlling the quantity of air supplied to the system. A drawback of this system is that not the reactant itself is adjusted, i.e. for example the oxygen, but rather the air is adjusted, which may contain varying concentrations of oxygen. This may give rise to the situation in which, despite the supply of air being restricted, the power output by the fuel cells remains unchanged since, coincidentally, the oxygen partial pressure of the air at the restriction point has risen simultaneously.
Published European Patent Application No. EP 0 828 303 A2, corresponding to U.S. Pat. No. 5,925,476, discloses a fuel-cell-operated generator system for current, in which fuel cell stacks which are supplied with process gas are used. In this system, in order to set a desired power and to determine a deviation from the set power, it is essentially checked whether the catalyst of the fuel cells has been poisoned. To do this, inter alia the temperature and the pressure of the process gases supplied are recorded and regulated. Furthermore, Published European Patent Application No. EP 0 595 503 A1, corresponding to U.S. Pat. No. 5,445,902, describes a method for operating a solid oxide fuel cell (SOFC) installation, in which the temperature management is substantially dependent on the conditions set.
It is accordingly an object of the invention to provide a method for controlling the power of a fuel cell stack, a method for controlling the power of a drive unit of an electric vehicle, and a fuel cell device which overcome the above-mentioned disadvantages of the heretofore-known methods and device of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for controlling a power of a fuel cell stack, the method includes the steps of:
supplying, via a feed line, fuel gas and air as process gases to a fuel cell stack;
measuring an oxygen partial pressure of a reaction gas in a cathode exhaust air;
comparing a set power with an actual power output by the fuel cell stack by correlating the oxygen partial pressure of the reaction gas with the actual power; and
using the oxygen partial pressure in the cathode exhaust air as a reference variable for setting a process gas mass flow rate, and setting an air mass flow rate accordingly with a control unit.
In other words, a method for controlling the power of a fuel cell stack which is supplied with fuel gas and air as process gases via a feed line, the partial pressure of a reaction gas being measured and a set power being compared with the actual power output by the fuel cell stack as a result of the partial pressure of reaction gas, which is present in the process gas, being correlated with the power output and the quantity of process gas supplied being set according to the requirements, wherein the oxygen partial pressure in the cathode exhaust air is measured and serves as a reference variable for setting the mass flow rate of process gas to be supplied, the mass flow rate of air supplied being set accordingly by a control unit.
With the objects of the invention in view there is also provided, a fuel cell device, including:
a fuel cell stack;
a process gas line, connected to the fuel cell stack, for supplying fuel gas and air to the fuel cell stack;
a cathode exhaust-gas line connected to the fuel cell stack;
a probe, provided in the cathode exhaust-gas line, for measuring an oxygen partial pressure of a reaction gas in a cathode exhaust air; and
a control unit, operatively connected to the probe, the control unit comparing a set power with an actual power output by the fuel cell stack by correlating the oxygen partial pressure of the reaction gas with the actual power, and the control unit using the oxygen partial pressure of the reaction gas in the cathode exhaust air as a reference variable for setting a process gas mass flow rate and for setting an air mass flow rate of the air to be supplied to the fuel cell stack.
According to another feature of the invention, data input units are operatively connected to the control unit, and at least one of the data input units is connected to the probe for measuring the oxygen partial pressure.
With the objects of the invention in view there is also provided, a method for controlling a power of a drive unit of an electric vehicle, the method includes the steps of:
supplying, via a feed line, fuel gas and air as process gases to a fuel cell stack of a drive unit of an electric vehicle;
measuring an oxygen partial pressure of a reaction gas in a cathode exhaust air;
comparing a set power with an actual power output by the drive unit by correlating the oxygen partial pressure of the reaction gas with the actual power; and
using the oxygen partial pressure in the cathode exhaust air as a reference variable for setting a process gas mass flow rate and for setting an air mass flow rate accordingly with a control unit.
With the objects of the invention in view there is also provided, a method for controlling a power of a drive unit of an electric vehicle, the method includes the steps of:
supplying, via a feed line, fuel gas and air as process gases to a fuel cell stack of a drive unit of an electric vehicle;
operating the fuel cell stack with air as an oxidant;
measuring, with a probe provided in a cathode exhaust-gas line, an oxygen partial pressure of a reaction gas in a cathode exhaust air;
comparing, with a control unit, a set power with an actual power output by the drive unit by correlating the oxygen partial pressure of the reaction gas with the actual power, and using the oxygen partial pressure in the cathode exhaust air as a reference variable for setting a process gas mass flow rate, and setting an air mass flow rate accordingly with the control unit; and
controlling a rotational speed of a motor for an air compressor for the fuel cell stack by taking into account, with the control unit, commands of a driver, data of an external-temperature measurement and an ambient pressure measurement, data concerning a mass flow rate of air currently supplied to the fuel cell stack, and data concerning the oxygen partial pressure in the cathode exhaust air.
The invention relates to a method for controlling the power of a fuel cell stack, in which the oxygen partial pressure in the cathode exhaust air is measured and serves as a reference variable for setting the mass flow rate of process gas to be supplied. In this method, through the use of a control unit the set power is compared with the actual power output by the fuel cell stack and the control unit takes into account the oxygen partial pressure which is still present in the exhaust gas when setting the air mass flow rate. In the associated device, the probe is provided in the cathode exhaust gas line for the purpose of measuring the oxygen partial pressure of the cathode exhaust air. The measured value is transmitted to the control unit and serves as a reference variable for setting the mass flow rate of air which is to be supplied to the fuel cell stack.
According to an advantageous configuration of the invention, further data, such as the ambient pressure and the outside temperature of the installation, is available to the electronic control unit and can be used to more accurately meter the mass flow rate of air to be supplied.
In the present case, the humidified and compressed gas/liquid mixture which is introduced into the fuel cell stack for conversion purposes is referred to as the process gas. One example of a process gas is humidified air, but the process gas may also be pure oxygen or pure hydrogen, as well as a water/methanol mixture. It is preferable to use air for the cathode-side process gas, the mass flow rate of which is set through the use of the electronic control unit.
The term reaction gas refers to the gas which is actively converted in the fuel cell. This is preferably pure oxygen, pure hydrogen or pure methanol, depending on the type of PEM (proton exchange membrane) fuel cell. The reaction gas has no inert gas and/or steam content. Therefore, the probe only measures the partial pressure of a gas in the gas/liquid mixture of the exhaust gas from the fuel cell.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for controlling the power of a fuel cell stack, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.